As a conventional electric motor, for example, one described in Patent Document 1 below is known. This electric motor has an inner rotor, a stator, and an outer rotor; the inner rotor has a columnar shape in which a plurality of permanent magnets extending slightly in a radial direction are disposed in the peripheral direction, the stator has a cylindrical shape in which a plurality of armatures are disposed in the peripheral direction and fixed by a resin mold, and the outer rotor is formed in a cylindrical shape by winding a coil around a core in which a plurality of rings are laminated, electric power not being supplied to this coil. Furthermore, the inner rotor, the stator, and the outer rotor are provided in order from the inside and are relatively rotatable.
In this electric motor, when electric power is supplied to the stator to thus generate a rotating magnetic field, the magnetic poles of the permanent magnets of the inner rotor are attracted to/repelled by the magnetic poles of the stator, the inner rotor thereby rotates in synchronization with the rotating magnetic field, and the outer rotor rotates out of synchronization with the rotating magnetic field by virtue of electromagnetic induction.
Furthermore, an arrangement in which a rotor of a permanent magnet type electric motor includes a first permanent magnet group and a second permanent magnet group that are adjacent to each other in the axial direction, the first and second permanent magnet groups each being formed by disposing a plurality of permanent magnets on an outer peripheral face of the rotor so that different magnetization directions alternate, and the direction of magnetization of a permanent magnet of the first permanent magnet group is different from that of an axially adjacent permanent magnet of the second permanent magnet group is known from Patent Document 2 below.
Furthermore, an arrangement in which a rotor of a stepping motor includes a first rotor portion to a fourth rotor portion arranged side by side in the axial direction, each rotor portion including a plurality of permanent magnets for which the polarity alternatingly reverses in the circumferential direction, the polarity of the permanent magnets is displaced by ½ pitch in the circumferential direction between the first rotor portion and the second rotor portion, the polarity of the permanent magnets is displaced by ½ pitch in the circumferential direction between the third rotor portion and the fourth rotor portion, and the polarity of the permanent magnets of the first and second rotor portions is displaced by ¼ pitch in the circumferential direction from the polarity of the permanent magnets of the third and fourth rotor portions is known from Patent Document 3 below (ref fifth embodiment in FIG. 12).
Patent Document 1: Japanese Patent Application Laid-open No. 11-341757
Patent Document 2: Japanese Patent Application Laid-open No. 2007-129869
Patent Document 3: Japanese Patent Application Laid-open No. 9-9602